This application claims the benefit of Japanese Patent Application No. 2003-005840, filed on Jan. 14, 2003; Japanese Patent Application No. 2003-106674, filed on Apr. 10, 2003; and Japanese Patent Application No. 2003-309297, filed on Sep. 1, 2003.
1. Field of the Invention
The present invention relates to a motor stator core made of non-oriented electrical steel sheets. In particular, the invention relates to a motor stator core of improved magnetizing feature in lower magnetic fields and a method of manufacturing the same.
2. Description of the Related Art
Recently, in view of environmental and resources problems, energy savings and improved efficiency have been increasingly demanded of electric equipment. The biggest request for motors is also an efficiency improvement, which in turn requires reductions of iron loss, copper loss, and mechanical loss. Such motors use a lamination of non-oriented electrical steel sheets for their core. It is well known that the core has a large impact on iron loss.
Motor cores are made of non-oriented electrical steel sheets, which are manufactured through the application of cold rolling, including one or more processes of annealing, to hot-rolled steel sheets that are adjusted to predetermined chemical compositions. Some of the manufacturing steps vary between semi-processed non-oriented electrical steel sheets and full-processed non-oriented electrical steel sheets. Semi-processed non-oriented electrical steel sheets are predicated on the application of stress-relief annealing after punching. The cold-rolled steel sheets are thus annealed at relatively low temperatures for a short time. The annealing also includes skin passed rolling in its final step. In contrast, full-processed non-oriented electrical steel sheets are not necessarily predicated on the stress-relief annealing of the core. The cold-rolled steel sheets are thus annealed at higher temperatures for a longer time than the semi-processed non-oriented electrical steel sheets are. To ease this annealing condition, the hot-rolled steel sheets are sometimes annealed.
By the way, the design concept of high-speed high-efficiency motors typified by electric vehicle motors, which are under intensive development and commercialization recently, is toward higher frequencies (400 to 600 Hz) and lower magnetic induction (1.0 to 0.5 T). A further improvement for lower iron loss is thus required of non-oriented electrical steel sheets, or motor core material, accordingly.
Motor power P is expressed by the following general equation:P=k×f×N×i×B×S,where k is a proportionality constant, f is a frequency, N is the number of windings, i is a current, B is a magnetic induction, and S is the sectional area of the core. From the foregoing equation, it is first shown that with consideration given to the miniaturization and weight reduction of the motor, the power P may be increased by raising the frequency f of the exciting current. Frequencies as high as 10 times or so the frequencies of commercial power sources are available recently owing to the invention of inverters. For this reason, the frequency f and the magnetic induction B have been evaluated for optimum solutions in the range of frequencies higher than those of the conventional commercial power sources. The feasible range of magnetic induction B at a given frequency f depends on the occurrence of anti-magnetizing force from the motor core material, or the non-oriented electrical steel sheets. Consequently, the frequency f is typically set within the range from 400 to 600 Hz, and the magnetic induction B the range from 1.0 to 0.5 T. In such numerical ranges, the magnetic induction B remains at a decreasing rate of ½ or so for 8× to 10× frequencies f. This promises a power increase of the order of f×B=4 to 5 times. The numerical ranges of the values k, N, i and S in the foregoing equation are automatically determined once the frequency f and the magnetic induction B are determined.
The iron loss W of a non-oriented electrical steel sheet, or motor core material, is expressed as:W=Wh+We,
where Wh is hysteresis loss which is given byWh=k1×f×B1.6,
We is eddy current loss which is given byWe=k2×(t2×f2×B2)/ρ, and                k1 and k2 are constants, t is a thickness, and ρ is a resistivity.For the sake of reduction in this iron loss, steel sheets have been reduced in thickness and increased in resistivity. Some of the most excellent non-oriented electrical steel sheets at present have thicknesses t as small as 0.20 mm or so and resistivities as high as or above 55 μΩ-cm.        
As a result of these measures for reducing iron loss by means of reduced thicknesses and increased resistivities, non-oriented electrical steel sheets have been considerably reduced in iron loss as compared to heretofore. Nevertheless, the reduction in iron loss resulting from the reduced thickness and increased resistivity of the electrical steel sheets is ascribable to a reduction of the eddy current loss out of the iron loss expressed by the foregoing equation. The hysteresis loss is not reduced by a reduction in thickness or an increase in resistivity. As the eddy current loss decreases, the ratio of the hysteresis loss to the entire iron loss increases relatively from conventional 70% or so to 90% or so. For future measures for a reduction in iron loss, it is thus becoming increasingly important to reduce the hysteresis loss.
The hysteresis loss of an electrical steel sheet has a close relationship with magnetic induction, and magnetizing feature can be improved to reduce the hysteresis loss. Consequently, improvernent in the magnetizing feature matters for the sake of a reduction in hysteresis loss. The magnetizing feature can be improved by making the crystal direction of an electrical steel sheet to be a random cubed direction. For concrete means, one of the inventors has developed a non-oriented electrical steel sheet which is switched from a conventional Si-rich composition to an Al—Mn rich composition to achieve both reduced iron loss and improved magnetizing feature (see Unexamined Japanese Patent Publication No. 2002-146490).
To fabricate a typical motor core, hoops of non-oriented electrical steel sheets oiled with punching oil are press-punched into a predetermined shape, and then a lamination of the resultant is firmly bonded by clumping or welding. Subsequently, heat treatment is performed to remove the adhering punching oil, followed by annealing.
The primary purpose of this annealing is to remove strains occurring at the time of punching as well as to promote the growth of crystal grains for improved magnetizing feature. It is semi-processed non-oriented electrical steel sheets that are predicated on the application of this stress-relief annealing to the core after punching. Conventionally, motor cores have been annealed under the condition that they are kept in a non-oxidizing or reducing atmosphere at soaking temperatures of approximately 750° C. for approximately 2 hours. Recently, it has been proposed to perform this kind of core annealing in a magnetic field. For example, Unexamined Japanese Patent Publication No. Hei 11-340030 describes a method in which the electrical steel pieces of a core to be excited in two or more directions inside are annealed in magnetic fields having the same directions as the directions of their excitation. Moreover, Unexamined Japanese Patent Publication No. Hei 11-341749 describes a method of annealing a core under the application of a magnetic field, wherein the magnetic field to be applied to the core is caused by a coil, and the core is heated for annealing by the heat generated by the coil or through the application of a high-frequency magnetic field from the coil.
The technology for applying heat treatment to metal in a magnetic field has been studied long. Even with Fe—Si alloys, however, it has been concluded in the 1960s that “heat treatment in a magnetic field has been developed for the purpose of improving Fe—Si alloys' permeability and produced excellent outcomes in laboratories, but of little practical value since it requires a lot of work and cost.” No practical use has thus been attempted so far.
Nevertheless, it is highly probable that annealing a motor stator core, or a subject matter of the present invention, in a magnetic field can improve magnetizing feature. Under the present circumstances, however, the relationship of the material properties and annealing condition to the magnetizing feature of the annealed core has not been clarified. Moreover, the magnetic annealing of a core proposed in the foregoing Unexamined Japanese Patent Publications Nos. Hei 11-340030 and Hei 11-341749 has made no reference in this regard. The inventors have also filed a patent application for a method and apparatus for annealing a motor stator core in a magnetic field as Japanese Patent Application No. 2002-156136, whereas this prior application has not provided sufficient resolution in this regard.
As described above, electric vehicle motors are currently adopting exciting currents of higher frequencies in the frequency range from 400 to 600 Hz to provide power higher than with the exciting currents of commercial power frequencies. Nevertheless, an increase in the frequency of the exciting current can lower the stator cores to ½ or so, or even below, in magnetic induction. Here, if the stator cores can be prevented from dropping in magnetic induction even under exciting currents of higher frequencies, it is possible to expect a further enhancement to the power of the electric vehicle motors at present, or a miniaturization of the same.